The invention relates to a method of cleaning an electronic filter during filtration and to an electric filter.
The emission system of an electric filter is formed of negatively charged emission electrodes and of separation electrodes in a zero position or of ground plates (the separation system, functioning as a positive pool). The gas to be purified of particles is fed through the emission system of the electric filter and the gas flows in the emission system between positively and negatively charged electrodes. Since a tension of approximately 100 kV typically exists between the positively and negatively charged electrodes, such a tension provides corona discharges between the electrodes. The corona discharges cause the particles, when they flow through such a corona discharge, to be mainly negatively charged and attached to the positively charged plates, whereas the positive particles are attached to the emission electrodes.
Different electric filter applications are used for instance in power stations, pulp mills and in various metallurgical processes, in which the electric filters tend to separate particles from the hot gases fed therethrough.
It is previously known in electrical filters to shake the separation electrodes at regular intervals when the electric filter is being used, i.e. during filtration, in order to purify the separation electrodes of the particles attached thereto during filtration. The idea is that the particles removed by shaking fall to the lower part of the electric filter that is preferably but not necessarily provided with a bottom hopper.
What becomes a problem is that when separation electrodes are shaken during filtration, some of the particles removed from the separation electrodes by shaking are conveyed from the electric filter, since a gas flow flows through the electric filter. Thus, what is known as “rapping loss” is created.
A solution to the problem is to close the gas flow flowing through the electric filter completely when the separation electrodes of the electric filter are shaken, but this causes a break in filtration. Another known solution to the problem is of the kind in which two electric filters are used in parallel and in which the gas flow is closed during shaking through the electric filter that is shaken.
Publication U.S. Pat. No. 3,988,130 discloses an electric filter that allows reducing the gas flow in gas channels adjoining the separation electrode to be shaken by shaking means when such a separation electrode to be shaken by the shaking means is shaken simultaneously allowing gas to flow through the other gas channels of the electric filter. Thus, the separation electrodes are allowed to be shaken while using the electric filter without having to stop the electric filter. In this solution, another gas flow is directed against the gas flow flowing in the gas channel so that the gas flow substantially stops in the gas channel concerned. When the separation electrode adjoining the gas channel is shaken, the particles removed from the separation electrode may fall freely for instance to the bottom hopper at the bottom of the electric filter. A problem associated with this prior art solution is that a fairly complicated and space-requiring solution is required to direct a second gas flow against the gas flow flowing in the gas channel.
Publication JP8187450 discloses another electric filter that allows reducing the gas flow in gas channels adjoining a separation electrode to be shaken by a shaking means when such a separation electrode to be shaken by the shaking means is shaken simultaneously allowing gas to flow through the other gas channels of the electric filter. This prior art solution comprises a moving-type curtain that can be moved in front of the up-flow end of the gas intervals, thus preventing the gas flow out of the gas channel. When the separation electrode adjoining the gas channel is shaken, the particles removed from the separation electrodes may freely fall for instance to a bottom hopper at the bottom of the electric filter. A problem in this solution is to move the moving-type curtain reliably in dirty conditions in the electric filter.